what we learned from christchurch taking the risk out ... - PlaceMakers

FEATURESMBIE – BUILDING & HOUSINGDirecting house repairs and rebuildsGuidance for rebuilding and repairing houses in the Canterbury green zone hasbeen updated and brought together in one folder, making it easier to navigateTechnical guidance was originallycreated in 2011 to support theCanterbury rebuild and reflect thescale and effects of liquefaction. Itgives technical advice on foundationsfor houses in the Canterbury greenzone; structural repairs; assessingearthquake damage to hillsideproperties and retaining walls;and guidelines for subdivisions.Properties in the flat areas of theCanterbury green zone were dividedinto three Technical Categories (TCs),according to the level of investigationand the appropriate foundationsrequired for each site. This helpeddirect scarce engineering resourcesto areas of greatest need.For the benefit of readers fromoutside of Canterbury, the TCs werebased on area-wide land damageand house damage assessmentsduring the earthquake sequenceand known sub-surface geotechnicalconditions.The TC categories are:TC1: Liquefaction damage is unlikelyin future large earthquakes. Standardresidential foundation assessmentand construction is appropriate.TC2: Liquefaction damage is possiblein future large earthquakes. Standardenhanced foundation repair and rebuildoptions, outlined in the MBIE guidance,are suitable.TC3: Liquefaction damage is possiblein future large earthquakes. Individualengineering assessment is neededto select the appropriate foundationrepair or rebuild option.The December 2012 updates focuson repairs to damaged foundations,new foundation options, superstructurerepairs and subdivisions.There is now updated guidance on repairing and replacing wall bracing elements and significant wall damageFOUNDATION OPTIONSSome guidance on TC1 and TC2foundation options has been clarifiedand there are new options for TC3.Guidance on deep pile foundationshas been updated, and revised pilehead details for deep timber, concreteand steel piles are included. Theguidance lists the criteria that need tobe met when using a deep piled raft.ASSESSING DAMAGE TOFOUNDATIONSThe first task for a building professionalor engineer is to assess the extentof damage to foundations damagedby earthquakes and consider whetherfoundations can be re-leveled andrepaired, or must be rebuilt.MBIE’s guidance says there are threeaspects to consider:• Whether the building has settled bydifferent amounts over its floor, and/or is tilting.• Whether the foundations have been‘stretched’.• Damage to specific parts of thefoundations.These aspects must be consideredin relation to each other. Re-levels orrebuilds are triggered by excessivedifferential settlements or floor stretches.If the building has settled excessively, thefoundations may need to be rebuilt, evenif damage to the piles or the slab doesnot exceed certain limits. If there are cracksin the perimeter wall of the foundations,but the building hasn’t settled, it may bepossible to repair the foundations.If the slope of the floor between two pointsmore than 2 m apart is more than 0.5%(10 mm over 2 m), or the variation in levelover the floor plan is more than 50 mm, itindicates the floor or foundation probablyneeds to be re-leveled or possibly rebuiltdepending on the degree of tilt.Cracks in ceramic floor tiles and distressin vinyl floor coverings and carpet areother indicators. Even if these limits arenot exceeded, floor and superstructurerepairs may be required.6www.mbie.govt.nz